Battery charge measurement and discharge reserve time prediction technique and apparatus

ABSTRACT

A method of testing one or more cells and parameterising the results in order to obtain a characteristic curve/function from which cell discharge reserve time can be predicted from cell voltage. The test involves obtaining a plurality of data points representing the voltage of a cell as a function of charge remaining, and parameterising the data points to obtain a function representing cell voltage and charge remaining. The function allows charge remaining to be calculated from cell voltage. The invention also provides for a device for measuring capacity and predicting discharge reserve time of a cell, the device including a voltage and current measuring means adapted to measure the voltage and load current of a cell. The device also includes a timing means so that a number of voltage and current data points can be obtained with respect of time. The data points are parameterised and the device produces a function relating charge remaining to cell voltage whereby the charge remaining can be determined by measuring the cell voltage.

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for predictingand gauging the depth of available energy in a battery orelectrochemical cell system. More particularly, although notexclusively, the present invention relates to methods and apparatus formeasuring battery capacity, charge remaining and reserve time.

BACKGROUND TO THE INVENTION

It has long been recognised that battery or cell capacity depends on anumber of factors. These include the batteries composition, geometry,discharge rate (i.e. load current), age, environmental temperature, endvoltage, service history (i.e. characteristics of the batteries lastcharge), discharge depth and time on float. The available capacity of abattery can be represented as a complex, non-linear function of theseparameters. The direct measurement of a number of these parameters todetermine battery capacity is either impractical or financiallyprohibitive. One of the most well known techniques for measuring thecapacity of a battery is known as a “discharge test” (IEEE Std 1188-1996; “IEEE Recommended Practice for Maintenance, Testing, and Replacementof Valve-Regulated Lead-Acid (VRLA) Batteries for StationaryApplications”). This procedure involves the full discharge of a batteryinto a stable load. A major disadvantage of this approach is that, inthe context of the batteries application (for example in atelecommunication system), the system being powered is vulnerable topower outages as the battery must be disconnected from the system duringits complete discharge. Further disadvantages include the necessity forbulky external loads, the need for backup power supplies and the labourinvolved in setting up and supervising the testing procedure.

Other techniques for measuring the battery capacity use methods wherebyparameters such as impedance, (Gary J. Markle, “AC Impedance Testing forValve Regulated Cell,” INTELEC 1992, 9-4), conductance, (Michael E. Troyet al, “Midpoint Conductance Technology Used in TelecommunicationStationary Standby Battery Applications. Part VI. Considerations forDeployment of Midpoint Conductance in Telecommunications PowerApplications,” INTELEC 1997, 29-4), or internal resistance (IsamuKurisawa and Masashi Iwata, “Internal Resistance and Deterioration ofVRLA Battery—Analysis of Internal Resistance Obtained by Direct CurrentMeasurement and its Application to VRLA Battery Monitoring Technique,”INTELEC 1997, 29-3: Glenn Alber and Marco W. Migilaro, “ImpedanceTesting—s it a Substitute for Capacity Testing,” INTELEC 1994, 10-1:Katsuhiko Yamamoto et al, “Deterioration Estimation Method for 200-AhSealed Lead-Acid Batteries,” NTT Review Vol. 7, No. 4, July 1995) arecorrelated with a capacity. These latter methods generally employ acomposite model based on a number of parameters. This model usuallyincorporates reference to cell resistance or impedance in determiningthe battery capacity (Jean Paul Cun et al, “The Experience of a UPSCompany in Advanced Battery Monitoring,” INTELEC 1996, 22-5: Petrick K.Ng et al, “Evaluation of a Reverse Time Prediction Algorithm for LeadAcid Battery”, INTELEC 1996, 616-21). These methods have generally beendeveloped for off-line applications and require the use of specialisedequipment. Although they have had some success, it is generallyconsidered in the art that these techniques are best suited foridentifying gross faults (Michael E. Troy et al, “Midpoint ConductanceTechnology Used in Telecommunication Stationary Standby BatteryApplications. Part VI. Considerations for Deployment of MidpointConductance in Telecommunications Power Applications,” INTELEC 1997,29-4: Katsuhiko Yamamoto et al, “Deterioration Estimation Method for200-Ah Sealed Lead-Acid Batteries,” NTT Review VoL. 7, No. 4, July1995), tracking battery age and making battery life time predictions(Gary J. Markle, “AC Impedance Testing for Valve Regulated Cell,”INTELEC 1992, 9-4: Katsuhiko Yamamoto et al, “Deterioration EstimationMethod for 200-Ah Sealed Lead-Acid Batteries,” NTT Review VoL. 7, No. 4,July 1995). A detailed short-term test of battery capacity measurementis still most effectively produced by the discharge test. Referringagain to the latter models discussed above, such models used for on-linecapacity measurements are often specific to particular cells and rely onmeasured parameters (Isamu Kurisawa and Masashi Iwata “CapacityEstimating Method of Lead-Acid Battery by Short- time Discharge”,INTELEC 1997, 483-90). Such techniques are therefore susceptible tomeasurement errors. Further, the number of parameters necessary toclassify an entire battery operation can become excessive making suchapproaches cumbersome and computationally complicated.

Alternative techniques for determining battery capacity have beenproposed which are based on either open circuit voltage or chargeaccumulation (Minoru Kozaki, and Toshihiko Yamazaki, “Remaining BatteryCapacity Meter and Method for Computing Remaining Capacity,” U.S. Pat.No. 5,691,078, Nov. 25, 1997).

In the context of telecommunications applications, the open circuitmethod is undesirable. Disconnecting the battery string from the powersupply system would leave the telecommunication system vulnerable toswitch failure and hence accidental isolation of the string from thesystem. Further, the charge accumulation approach requires long termmonitoring of the battery (or battery string) and is dependent onknowing an accurate initial value of the battery capacity. Any initialerror would affect the results of the rest of the monitoring activity.For this reason, this latter approach is considered unreliable.

It is accordingly an object of the present invention to provide a methodand apparatus which allows for an accurate measurement of a batteriescharge remaining (within the constraints of the application to which thebattery is to be put), which avoids or at least ameliorates a number ofthe above mentioned problems, or at least provides the public with auseful choice.

DISCLOSURE OF THE INVENTION

In one aspect, the present invention provides for a method oftesting/characterising one or more cells including the steps of:

obtaining a plurality of data points representing the directrelationship between voltage and charge remaining during an initialdischarge of one or more cells; and,

parameterising the data points to obtain a function representing voltageand charge remaining, the function terminating at an end voltagecorresponding substantially to a voltage level at which the cell(s)is/are considered to be exhausted, and whereby during a subsequentdischarge the function allows the charge remaining to be estimateddirectly from the cell(s) voltage.

After the parameterisation there may be a further step of calculatingthe discharge reserve time. Preferably the discharge reserve time iscalculated by dividing the charge remaining by either a constant powerdischarge rate or a constant current discharge rate.

Additionally there may be yet a further step whereby a fully chargedcell or cells is subjected to a partial discharge, the charge releasedduring the partial discharge being added to the charge remaining toobtain a measurement of capacity.

Preferably the partial discharge is one that is long enough to avoid theCoup de Fouet region, but is much shorter than a complete discharge ofthe cell or cells.

The steps can correspond to measuring the cell(s) voltage and currentover specific time intervals.

The parameterisation can be effected by means of collecting data pointsequidistant in the voltage domain, a least squares fit, interpolationand/or extrapolation, or an analytical approach adapted to target thebest fit to the data points.

A number of data points for a set level of measurement accuracy can beobtained and parameterised.

Preferably the data points are selected over intervals selected so as tominimise the errors inherent in the parameterisation process.

Preferably any or all of the steps may be repeated due to changes incell characteristic from ageing, environmental and usage conditions.

Preferably the decision to repeat the steps is determined by comparisonof a state of change of the cell(s) derived from a previous test and theactual state of charge of the cell(s).

In a further aspect, the invention provides for a battery chargeremaining and capacity measurement and discharge reserve time predictiondevice including:

a voltage measurement means adapted to measure the voltage of a cell orcells;

a current measurement means adapted to measure the present load on thecell or cells;

a timing means adapted so that a substantially simultaneous measurementof voltage, current and time in respect of the cell or cells can beperformed, thereby allowing the collection of a plurality of data pointsrelating to the direct relation between cell voltage and chargeremaining; and

a processing means adapted to produce a curve/function directly relatingcharge remaining to cell voltage during an initial discharge of one ormore cells, the curve/function terminating at an end voltagecorresponding substantially to a voltage level at which the cell(s)is/are considered to be exhausted, and whereby during a subsequentdischarge the curve/function allows the processing means to estimatecharge remaining directly from the cell(s) voltage.

The device can include a microprocessor adapted to manipulate thevoltage, time and current to provide data points representing thevoltage as a function of charge remaining wherein the charge remainingis expressed in amp/hours.

The device can be adapted to calculate the discharge rate of a cell orcells, the device using the charge remaining and a discharge rate todetermine the discharge reserve time. Preferably the discharge rate iscalculated for either constant power or constant current discharge, andthe discharge reserve time is expressed in hours and fractions of anhour.

The device can include a discharge means, the discharge means beingadapted to at least partially discharge a cell or cells and measure thecharge released from said cell or cells during the discharge, the cellor cells capacity being derived from the charge released during thedischarge and the charge remaining.

The device may incorporate an output means adapted to graphically,numerically or otherwise indicate, in real time, the charge remaining,capacity measurement and/or discharge reserve time of the cell or cellsbeing measured.

Preferably the device includes a means adapted to, at the initiation ofa user, measure the data points and effect a parameterisationautomatically.

The device can be further adapted to incorporated means for sensingvariations in the environmental conditions in which the cell or cellsare used and be further adapted so that in response to predeterminedcriteria, the device remeasures the data points and establishes anupdated parameterisation.

Preferably the device may output the charge remaining, capacitymeasurement and/or discharge reserve time of the cell or cellsconstantly.

Alternatively the charge remaining, capacity measurement and/ordischarge reserve time may be output in response to a user activation orrequest.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example and withreference to the Figures in which:

FIG. 1: illustrates schematically a testing arrangement for strings ofcells;

FIG. 2: illustrates a curve representing the charge remaining domaindischarge characteristic;

FIGS. 3a and b: illustrate time and charge remaining domain dischargecharacteristics of six different cells of the same type;

FIGS. 4a and b: illustrate time and charge remaining domain dischargecharacteristics of different cells of the same type at differentdischarge rates;

FIGS. 5a and b: illustrate the effects of discharge rate switchingduring the discharge;

FIGS. 6a and b: illustrate time and charge remaining domain dischargecharacteristics of six different cells of the same type at differenttemperatures;

FIGS. 7a and b: illustrate time and charge remaining domain dischargecharacteristics of eight different cells of the same type with differentdischarge rate and temperature combinations;

FIGS. 8a and b: illustrate time and charge remaining domain dischargecharacteristics of four different cells of the same type with differentcharge levels;

FIG. 9: illustrates actual and measured charge remaining and measurementerror for a new forty-eight volt string of cells discharged at a rate of68 amps;

FIG. 10: illustrates the discharge characteristics of two cells, onecomplying and one not complying with the nominal characteristic ; and

FIG. 11: illustrates the ratio of actual and measured charge remainingfor the compliant and noncompliant cells shown in FIG. 10.

As a preliminary point, the following description will be givenprimarily in the context of telecommunications power supply systems.This is not to be construed as a limitation as there exist otherapplications to which the present invention is suited. These will bediscussed in more details below.

Generally environmental parameters of telecommunication systems arecontrolled within specified limits. Given these limits, the minimum busvoltage should protect the system from being driven into undervoltageand provide at least the minimum voltage at which the telecommunicationsystem will operate.

In the context of a digital telecommunication system these requirementstranslate into an acceptable load current variation of ±5%. Wider loadvariations are anticipated in analogue systems. A further factor whichdefines the degree of accuracy required in the measurement of batterycharge remaining, is that most telecommunication sites are located inair conditioned environments which are kept at an optimal temperature.

To best illustrate the invention, various data are discussed below withreference to the method and apparatus of the invention. The data shownin FIGS. 3 to 8 were collected with a test circuit as illustrated inFIG. 1.

Referring to FIG. 1, the batteries were charged using a power systemconsisting of three 24V, 120 A rectifier modules. The cells weredischarged using a 2500 W electronic load. The battery data wereacquired using a high-resolution data acquisition unit. Although notrepresentative of a practical implementation of the present invention,this schematic circuit serves to illustrate and support the followingdescription.

The circuit shown in FIG. 1 was used to acquire discharge rates,temperatures and voltages. The data thus obtained was analyzed on apersonal computer using an appropriate user interface. To gauge thevariation and predictability of the method of the present invention atdifferent temperatures, the test system was contained inside anenvironmental chamber.

The discharge rate was measured using a calibrated 150 A, 75 mV currentshunt. The test system exhibited a current resolution of better than 1mA and a voltage resolution of better than 1 mV at an acquisitionfrequency of up to 1 Hz. It was found that a sampling rate of 12 secondswas sufficient to cover most of the key features involved in theanalysis of this data.

As noted above, it is known to monitor battery discharge behavior bylooking at the discharge voltage/time characteristics of a cell. Such acharacteristic curve would only represent a single operating conditionwhich is specific to that particular discharge regime. Therefore, usingsuch a characteristic curve to perform charge remaining calculationswould require a number of adjustment factors to allow for changes inparameters such as temperature, discharge rate, cell type etc. It isknown to supply correction factors for these parameters. However, theseare only average estimated values and may not be sufficiently accuratefor effective measurement.

According to the present invention, it has been found that replacing thedischarge voltage/time characteristic with discharge voltage/chargeremaining characteristic renders the characteristic curve lessvulnerable to parameter changes such as those noted above.

FIG. 2 illustrates a discharge voltage characteristic as a function ofthe charge remaining where charge remaining is measured in Ah(ampere-hours).

With reference to FIG. 2, the end voltage has been identified with zerocharge remaining. The selection of this condition is governed by therequirements of telecommunication systems in general. However, it ispossible that a different end voltage may be used depending on theparticular application of the invention.

Tying the discharge voltage/charge remaining characteristic curve to anend voltage results in the discharge curves being repeatable and followa similar linear trend within a fairly narrow tolerance. This techniqueprovides for a high degree of predictability of the charge remainingusing the discharge voltage.

The following discussion will illustrate the sensitivity of thischaracteristic to a number of the primary battery operating parameters,being temperature, discharge rate and battery type.

The discharge test results illustrated in FIGS. 3 to 9 were obtainedfrom tests on Hawker Energy 2RG310 2V, 310 Ah cells which were fouryears old. Unless otherwise indicated, the environmental temperature was20° C. and the discharge rate used was C/3 or 100 A.

With reference to FIGS. 3a and 3 b, six cells were fully charged andthen discharged at a rate of C/3. The curves are compared to a nominaldischarge curve that has been generated at a C/10 discharge rate.Different cells may exhibit different charge remaining reflecting thestate of charge they retain. All the cells, however, show completeconsistency with the nominal curve. As can been seen from FIG. 3a, thedischarge curves exhibit an initial deviation which is due to the “Coupde Fouet region”. Different discharge curves exhibit marginal deviationsfrom each other. The deviation exhibited by the characteristic curveswas found to be within a given tolerance band governed by therequirements of the particular application of these cells. As can beseen from FIG. 3b, transforming the characteristic curve to the chargeremaining domain results in a tighter tolerance band compared to thetime base characteristic (see FIG. 3a). FIGS. 4a and 4 b illustrate theresults of measuring discharge voltage versus charge remainingcharacteristic against different discharge rates. The rates selectedwere C/10, C/6, C/4, C/4, C/2.5 and C/2. Time based (i.e. prior art) andcharge remaining based representations are shown. FIG. 4b illustratesthat all of the discharge curves obtained in accordance with theinvention are bounded within the specified tolerance band.

FIG. 5 illustrates the effect of changing the discharge rate duringdischarge. Switching the discharge rate from high to low causes the cellto release more charge and hence to jump back along the nominaldischarge curve (see the arrow in FIG. 5b). Alternatively, switchingfrom a low rate to a high rate (see the arrow in FIG. 5a) reduces thecharge remaining. This causes the trend of the cell to jump forwardalong the nominal discharge curve. Both of these examples exhibitconsistency in following the nominal discharge curve in spite of changesin charge remaining caused by variations in the discharge rate.

FIGS. 6a and 6 b illustrate the results in testing the discharge voltageversus charge remaining at a number of different temperatures. Giventhat most telecommunication installations incorporate air-conditionedenvironments which are set at between 15 and 25° C., the characteristiccurves obtained according to the invention for such a temperature bandfall within the required tolerances.

FIGS. 7a and 7 b illustrate test results relating the discharge voltageto the charge remaining for variations in both discharge rate andtemperature. This test is aimed at verifying the robustness of therelationship for multiple parameter variations. Referring to FIG. 7b, itcan be seen that the majority of the discharge curves are consistentwith the nominal discharge curve within the required tolerance range.

Finally, FIGS. 8a and 8 b illustrate the results of the discharge offour cells with different initial levels of charge remaining. The cellsare discharged to different depths and then left to settle for one hour.After that time, the cell is discharged to the end voltage. The fourcells therefore contain different charge remaining at the start of theirdischarge. FIG. 8a illustrates the discharge characteristic in the timedomain. The discharge characteristics in the charge remaining domain areshown in

FIG. 8b. The results illustrate a good degree of consistency infollowing the trend of the nominal discharge curve within the specifiedtolerance band.

As can be seen from the results shown in FIGS. 3 to 8, it has been foundthat there is a robust relationship between the battery dischargevoltage and the charge remaining. By treating the battery or cell asessentially a “black box”, a characterising curve may be utilised formeasuring battery charge remaining during its discharge.

The technique may be implemented by representing the characteristiccurve by a table of discharge voltage/charge remaining data. Duringdischarge of the cell, this data is utilised to calculate the chargeremaining at each discharge voltage or continuously by means of a realtime determination.

To implement the technique practically, a number of measurements ofdischarge voltage versus charge remaining are made. These data are thenparameterised in the form of a curve whereby for any given dischargevoltage, a charge remaining value can be derived from the curve.Generally, the charge remaining values are given as percentages relatedto the rated cell capacity. As an example, measurement of the chargeremaining for a string of 24 Hawker Energy 2HI1275 2V cells is shown inFIG. 9. The results show a percentage error of less than 5% throughoutthe discharge of the cells. The quality of the results may be partiallyattributed to the consistency of new cells and partly due to theaveraging effect of the string.

In implementing the present technique in a telecommunications powersupply situation, it has been found that only six data points arenecessary to provide the required degree of predictability within thetolerances defined by such applications. The selection of six datapoints to represent the discharge data represents a trade off betweenaccuracy and data storage. This also satisfies the IEEE requirement fora minimum of five points to represent a discharge (IEEE Std 1188-1996;“IEEE Recommended Practice for Maintenance, Testing, and Replacement ofValve-Regulated Lead-Acid (VRLA) Batteries for StationaryApplications”).

In addition, six data points were found to be an acceptable number ofpoints for user actuated processes whereby a manual discharge table isgenerated by a user. This limitation was imposed as it was consideredthat increasing the number of data points could increase the chances ofmanual data entry errors. Once the six data points are obtained, alinear interpolation is used to derive the charge remaining. From thiscurve, charge remaining can be calculated for any arbitrary dischargevoltage.

In a practical implementation, a device implementing the presentinvention may incorporate auto-discharge table generation functionalityas well as manual table generation. The auto-discharge table generationfacility would enable a user to update the battery characteristic tableat a random or predefined time interval. The decision whether tore-characterise the battery may depend on the environmental conditionsin which the battery is stored or changes in both the environmentalconditions and the anticipated battery characteristic as the batteryages.

Even though a reliable measurement of the charge remaining can begathered using the voltage, a drastic change in battery behavior maytake place due to thermal stress or for some other operational reason.In this case a re-characterisation of the nominal discharge curve may berequired. The need for re-characterisation can be raised through acrosscheck between the rate of change of measured charge remaining withthe rate of change of actual charge remaining, which is equivalent tothe rate of change of charge released. An example is given in FIGS. 10and 11. FIG. 10 illustrates a nominal (16 A) discharge, and dischargesof a complying and non-complying (both at 60 A) cell from within anOldham 6RG180 mono-bloc. FIG. 11 illustrates the ratio of the rate ofchange of measured and actual charge remaining for the two 60 Adischarges. Ideally these curves should be equal to unity throughout thedischarge. However, the curve associated with the non-complying cellclearly diverges from unity, highlighting the necessity forre-characterisation. This technique of tracking the rate of change inmeasured charge remaining might also be used as an indicator for batteryreplacement or simply as an element to enable tuning of the chargeremaining measurement method. The test for the need forre-characterisation could be conducted in a real time mode during anyintentional or operational discharge.

Thus the present invention provides a highly robust method and apparatusfor charge remaining measurement during battery discharge. It has beenfound that a measurement accuracy of better than 10% is obtained andthis is within the constraints required by telecommunicationsapplications. The invention has shown to be valid for different cells ofthe same type, different discharge rates, environmental temperatures andlevel of charge. An implementation of the present technique may use anautomatic test for generating the required discharge table oralternatively this may be generated manually by means of userinteraction.

Once a measurement of charge remaining has been made the dischargereserve time can be calculated. This is usually performed by dividingthe charge remaining by the discharge rate and could be calculated foreither constant power or constant current discharge. This results in ameasurement of discharge reserve time in hours and fractions of an hour.

In addition to obtaining measurements of battery charge remaining thistechnique can be used to obtain measurements of battery capacity. Thisis performed by conducting a brief discharge of the fully chargedbattery and obtaining a measurement of the charge remaining, A briefdischarge in this context is one which is long enough to avoid the Coupde Fouet region but which is much shorter than a complete discharge tothe designated system end voltage. Once the measurement of the chargeremaining is obtained it is added to that charge released during thebrief discharge to obtain a measurement of the battery capacity.

It is envisaged that there exists other applications particularly suitedfor this battery monitoring technique. In particular, electric vehiclesprovide an ideal application as they routinely exhibit a large range ofload currents and hence data over a wide part of the cellscharacteristic discharge curve can be obtained as the user is drivingthe electric vehicle. In such an application, a device according to thepresent invention would be analogous to a fuel gauge showing distanceremaining at a specific speed and be constantly updateable according tomeasurements made as described above. Other applications of the presentinvention exist in general power supply contexts and these will be clearto one skilled in the art as would their implementation.

Referring to the implementation of the present invention, the dataprocessing aspects of the present invention may be carried out insoftware. To this end, an additional advantage of the present inventionis that the signal processing inputs are likely to be already existingin the power supply system. Therefore, the invention may be implementedeasily without the addition of substantial hardware other than thatassociated with the display electronics or telemetry as may be requiredin the particular application.

Where in the aforegoing description reference has been made to elementsor integers having known equivalents, then such equivalents are includedas if they were individually set forth.

Although the invention has been described by way of example and withreference to particular embodiments, it is to be understood thatmodifications and/or improvements may be made without departing from thescope or spirit of the attached claims.

What is claimed is:
 1. A method of calculating the discharge reservetime of one or more test cells including the steps of: obtaining aplurality of data points representing the direct relationship betweenvoltage and charge remaining during an initial discharge of one or morenominal cells; parameterising the data points to obtain a functionrepresenting charge remaining as a function of a single voltagevariable, the function terminating at an end voltage correspondingsubstantially to a voltage level at which the nominal cell(s) is/areconsidered to be exhausted; estimating charge remaining directly fromthe cell voltage of said test cell(s) during a discharge in accordancewith said function; and calculating the discharge reserve time of saidtest cell(s) from said estimated charge remaining.
 2. A method asclaimed in claim 1 wherein said test cells are different from saidnominal cells.
 3. A method as claimed in claim 1 wherein the dischargereserve time is calculated by dividing the charge remaining by aconstant power discharge rate.
 4. A method as claimed in claim 1 whereinthe discharge reserve time is calculated by dividing the chargeremaining by a constant current discharge rate.
 5. A method as claimedin claim 1 wherein a fully charged cell or cells is/are subjected to apartial discharge, the charge released during the partial dischargebeing added to the charge remaining to obtain a measurement of capacity.6. A method as claimed in claim 1 wherein a fully charged cell or cellsis/are subjected to a partial discharge, the charge released during thepartial discharge being added to the charge remaining to obtain ameausrement of capacity, and wherein the partial discharge is one thatis long enough to avoid the Coup de Fouet region, but is much shorterthan a complete discharge of the cell or cells.
 7. A method as claimedin claim 1 wherein the data points are obtained by measuring the cell(s)voltage and current over specific time intervals.
 8. A method as claimedin claim 1 wherein the parameterisation is effected by means ofcollecting data points equidistant in the voltage domain, a leastsquares fit, interpolation, extrapolation, or an analytical approachadapted to target the best fit to the characteristics.
 9. A method asclaimed in claim 1 wherein a minimum number of data points sufficientfor a set level of measurement accuracy are obtained and parameterised.10. A method as claimed in claim 1 wherein the data points are selectedover intervals selected so as to minimise the errors inherent in theparameterisation process.
 11. A method as claimed claim 1 wherein thesteps are repeated due to changes in cell characteristic from ageing,environmental and usage conditions.
 12. A method as claimed in claim 1wherein the steps are repeated due to changes in cell characteristicsfrom aging, environmental and usage conditions, and wherein the decisionto repeat the steps is determined by comparison of the change in chargeremaining measured for a cell(s) derived from a previouscharacterisation and the change in actual charge remaining of thecell(s).
 13. A battery charge remaining and capacity measurement anddischarge reserve time prediction device including: a voltagemeasurement means adapted to measure the voltage of a cell or cells; acurrent measurement means adapted to measure the present load on thecell or cells; a timing means adapted so that a substantiallysimultaneous measurement of voltage, current and time in respect of thecell or cells can be performed, thereby allowing the collection of aplurality of data points relating to the direct relationship betweencell voltage and charge remaining; and a processing means adapted toproduce a curve/function relating charge remaining as a single-variablefunction of cell voltage during an initial discharge of one or morecells, the curve/function terminating at an end voltage correspondingsubstantially to a voltage level at which the cell(s) is/are consideredto be exhausted, and whereby during a subsequent discharge thecurve/function allows the processing means to estimate charge remainingdirectly from the cell(s) voltage; and wherein the device is adapted tocalculate the discharge rate of a cell or cells, the device using thecharge remaining and discharge rate to determine the discharge reservetime.
 14. A device as claimed in claim 13 which includes amicroprocessor adapted to manipulate the voltage, time and current toprovide data points representing the charge remaining as a function ofvoltage, wherein the charge remaining is expressed in amp/hours.
 15. Adevice as claimed in claim 13 adapted to calculate the discharge rate ofa cell or cells, the device using the charge remaining and dischargerate to determine the discharge reserve time, wherein the discharge rateis calculated for either constant power or constant current discharge,and the discharge reserve time is expressed in hours and fractions of anhour.
 16. A device as claimed in claim 13 which further includes adischarge means, the discharge means being adapted to at least partiallydischarge the cell or cells and measure the charge released from saidcell or cells during the discharge, the cell or cells capacity beingderived from the charge released during the discharge and the chargeremaining.
 17. A device as claimed in claim 13 which includes an outputmeans adapted to graphically, numerically or otherwise indicate, in realtime, the charge remaining, capacity measurement and/or dischargereserve time of the cell or cells being measured.
 18. A device asclaimed in claim 13 adapted to, at the initiation of a user, measure thedata points and effect the parameterisation automatically.
 19. A deviceas claimed in claim 13 further adapted to include a means for sensingvariations in the environmental conditions in which the cell or cellsare used and further adapted so that, in response to predeterminedcriteria, the device remeasures the data points and establishes anupdated parameterisation.
 20. A device as claimed in claim 13 whereinthe device outputs the charge remaining, capacity measurement and/ordischarge reserve time of the cell or cells constantly.
 21. A device asclaimed in claim 13 wherein the charge remaining, capacity measurementand/or discharge reserve time may be output in response to a useractivation or request.
 22. A method of characterising one or more testcells including the steps of: obtaining a plurality of data pointsrepresenting a relationship between voltage and absolute chargeremaining during an initial discharge of one or more nominal cells; andparameterising the data points to obtain a function representingabsolute charge remaining as a function of a single voltage variable,the function terminating at an end voltage corresponding substantiallyto a voltage level at which the nominal cell(s) is/are considered to beexhausted, and whereby during a subsequent discharge the function allowsthe absolute charge remaining to be estimated from the cell(s) voltage.23. A method as claimed in claim 22 wherein a fully charged cell orcells is/are subjected to a partial discharge, the charge releasedduring the partial discharge being added to the charge remaining toobtain a measurement of capacity.
 24. A method as claimed in claim 22wherein a fully charged cell or cells is/are subjected to a partialdischarge, the charge released during the partial discharge being addedto the charge remaining to obtain a measurement of capacity wherein thepartial discharge is one that is long enough to avoid the Coup de Fouetregion, but is much shorter than a complete discharge of the cell orcells.
 25. A method as claimed in claim 22 wherein the data points areobtained by measuring the cell(s) voltage and current over specific timeintervals.
 26. A method as claimed in claim 22 wherein theparameterisation is effected by means of collecting data pointsequidistant in the voltage domain, a least squares fit, interpolation,extrapolation, or an analytical approach adapted to target the best fitto the characteristics.
 27. A method as claimed in claim 22 wherein aminimum number of data points sufficient for a set level of measurementaccuracy are obtained and parameterized.
 28. A method as claimed inclaim 22 wherein the data points are selected over intervals selected soas to minimise the errors inherent in the parameterisation process. 29.A method as claimed in claim 22 wherein the steps are repeated due tochanges in cell characteristic from ageing, environmental and usageconditions.
 30. A method as claimed in claim 22 wherein the steps arerepeated due to changes in cell characteristic from ageing,environmental and usage conditions, and wherein the decision to repeatthe steps is determined by comparison of the change in charge remainingmeasured for a cell(s) derived from a previous characterisation and thechange in actual charge remaining of the cell(s).
 31. A battery chargeremaining and capacity measurement and discharge reserve time predictiondevice including: a voltage measurement means adapted to measure thevoltage of a cell or cells; a current measurement means adapted tomeasure the present load on the cell or cells; a timing means adapted sothat a substantially simultaneous measurement of voltage, current andtime in respect of the cell or cells can be performed, thereby allowingthe collection of a plurality of data points relating to the directrelationship between cell voltage and charge remaining; and a processingmeans adapted to produce a curve/function relating absolute chargeremaining as a single-variable function of cell voltage during aninitial discharge of one or more cells, the curve/function terminatingat an end voltage corresponding substantially to a voltage level atwhich the cell(s) is/are considered to be exhausted, and whereby duringa subsequent discharge the curve/function allows the processing means toestimate absolute charge remaining directly from the cell(s) voltage.32. A device as claimed in claim 31 which includes a microprocessoradapted to manipulate the voltage, time and current to provide datapoints representing the charge remaining as a function of voltage,wherein the charge remaining is expressed in amp/hours.
 33. A device asclaimed in claim 31 which further includes a discharge means, thedischarge means being adapted to at least partially discharge the cellor cells and measure the charge released from said cell or cells duringthe discharge, the cell or cells capacity being derived from the chargereleased during the discharge and the charge remaining.
 34. A device asclaimed in claim 31 which includes an output means adapted tographically, numerically or otherwise indicate, in real time, the chargeremaining, capacity measurement and/or discharge reserve time of thecell or cells being measured.
 35. A device as claimed in claim 31,adapted to, at the initiation of a user, measure the data points andeffect the parameterisation automatically.
 36. A device as claimed inclaim 31 further adapted to include a means for sensing variations inthe environmental conditions in which the cell or cells are used andfurther adapted so that, in response to predetermined criteria, thedevice remeasures the data points and establishes an updatedparameterisation.
 37. A device as claimed in claim 31 wherein the deviceoutputs the charge remaining, capacity measurement and/or dischargereserve time of the cell or cells constantly.
 38. A device as claimed inclaim 31 wherein the charge remaining, capacity measurement and/ordischarge reserve time may be output in response to a user activation orrequest.